Ethylene polymers of improved glass, transparency &amp; heat-sealability containing a zinc salt of a fatty acid

ABSTRACT

Ethylene polymers having improved gloss and decreased haze which have incorporated therein from about 20 to about 400 parts per million of a zinc salt of a fatty acid having 7-22 carbon atoms. These polymers exhibit better printability after surface treatment than the same polymers without the fatty acid zinc salt and exhibit better heat sealability than do polymers containing more than the specified amounts of fatty acid zinc salt.

nited States Patent Smith [4 1 Apr. 18, 1972 1 ETI-IYLENE POLYMERS OF IMPROVED References Cited GLASS, TRANSPARENCY & HEAT- UNI TED STATES PATENTS SEALABILITY CONTAINING A ZINC s A F 2,462,331 2/1949 Myers ..260/23 3,261,900 7/1966 Spillers ..264/ 140 [72] Inventor: Vernon J. Smith, Lake Jackson, Tex. 3,367,926 2/1968 Voeks ..260/93.5 [73] Assignee: 'lsll' ehDow Chemical Company, Midland, Primary Examiner Donald Czaja Assistant Examiner-H. S. Cockeram [22] Filed: Sept. 25, 1970 Attomey-Griswold and Burdick, R. G. Waterman, L. .l. Dan- 1 pp 75 675 kert and M. S. Jenkins Related US. Application Data [571 I ABSTRACT Ethylene polymers having improved gloss and decreased haze [63] ggg ggggaz gg of Sen which have incorporated therein from about 20 to about 400 parts per million of a zinc salt of a fatty acid having 7-22 car- 52 US'CI. I M26003 2 0/ 17 2 0/949 GD 50!! atoms. These polymersexhibit better printability after [51] MC! u k308i 29/0LC03f45/00 surface treatment than the same polymers without the fatty 581 Field of Search ..260/23 H acid Zinc Salt and exhibit better heat Sealability than do polymers containing more than the specified amounts of fatty acid zinc salt.

8 Claims, No Drawings l ETIIYLENEPOLYMERS F IMPROVED GLASS, TRANSPARENCY & HEAT-SEALABEITYCONTAINING A ZINC SALT OF A FATTY ACID This application is a continuation in part of our co-pending application Ser. No. 754,959, filed Aug. 23, 1968, now abandoned.

BACKGROUND OF THE'INVENTION .Films of ethylene polymers are well known to be useful as bread wrapping, food bags, and in general packaging applications. It is desirable "to improve the gloss of these films to increase their appearance to the consumer and at the same time improve or maintain the susceptibility of the films to be rendered printable by surface treatment, such as by electrostatic treatment with a corona arc. "It is known that metal saltsof fatty acids can be added to ethylene polymers to improve the slip properties, to reduce blocking, for enhancing the processability, orfor improving clarity. The amounts used in the prior art, e.g., U.S. Pat. Nos. 3,367,926, 2,462,331 and 3,261,900, were in excess of 500 parts per million (p.p.rn. in order to achieve the desired resultsfUnfortunately films containing such quantities of the foregoing metal salts have poor heat-scalability.

SUMMARY OF THE INVENTION In accordance with the present invention, ethylene polymershaving improved surface gloss and haze and good heat sealability are made by incorporating into the polymer a zinc salt of a fatty acid having 7 to 22 carbon atoms. More particularly, the present invention is directed to improved ethylene polymers having incorporated therein from about 20 to about 400 parts per million (p.p.m.) of a zinc salt'of a monocarboxylic aliphatic saturated or unsaturatedacidhaving a chain length of 7 to 22 carbon atoms. Apreferred chain length range for the carboxylic acid is 16 to 20 carbon atoms.

The ethylene polymers'having from about 50 to about 400 p.p.m. of the fatty acid zinc salt blended therein are preferred and a fatty acid zinc salt concentration of from about 100 to about 350. p.p.m. is most preferred.

' Exemplary of the fatty acid zinc salts which can beused in thepolymers of this invention are the zinc salts of heptylic, caprylic, capric, pelargonic, lauric, myris'tic, palmitic, stearic, arachidic, and behenic as well as their unsaturated analogs such as oleic, ricinoleic, etc. Of the foregoing zinc stearate is the preferred additive. Commercially available zinc stearate having normally occurring impurities is sufficient for the purposes of this invention.

The ethylene polymers usedain this invention include polyethylene, blends of polyethylene, copolymers of ethylene and aliphatic, ethylenically unsaturated carboxylic esters, ethylene terpolymers comprised of ethylene, an ethylenically unsaturated carboxylic ester and an ethylenically unsaturated hydrocarbon, and mixtures'thereof. Polyethylene and blends of polyethylene are the most advantageously affected by the practice of this invention. Illustrative of the aliphatic, ethylenically unsaturated carboxylic esters which may be copolymei'ized with ethylene are the acrylic andmethacrylic esters and the vinyl esters of monobasic alkanoic acids. Specific' examples of such esters are methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butylacrylate, isobutyl acrylate, n-hexyl acrylate, cyclohexyl acrylate, 2- ethyl-hexyl acrylate, methyl methacrylate,ethyl methacrylate, n-propyl rn'ethacrylate, vinyl propionate, vinyl n-butyrate, vinyl isobutyrate and the like. Ethylene polymers having'a density within the rangeof about 0.9l0'to about 0.970 and a melt index in the range of about 0.01 to, about 20.0 are preferred.

1 Examples of the unsaturated hydrocarbons-which may be used to form the ethylene terpolymers include propylene, butene-l, pentene-l and the like. Especially useful terpolymers are terpolymers containing about 0.5-per cent to about 20 per cent by weight of an ethylenically unsaturatedmonocarboxylic acid ester such'as vinyl acetate, up to about 2 per cent of another copolymerizable ethylenically unsaturated hydrocarbon, especiallypropylen'e, the balance of the terpolymer com position, i.e., from about per cent to about 99 per cent by weight, of ethylene, all percentages being based on the total copolymer weight. These 'terpolymers are described in patent US. Pat. No. 3,326,840 issued to H. E. Ross and W. H. Wharton.

While the compositions of the present invention consist substa'ntially of ethylene polymers and zinc salts'of fatty acids, small amounts of conventional additives and modifiers known tobe useful in the plastic compounding art such as slip agents, anti-block agents, antioxidants, stabilizers, antistatic agents, pigments, waxes and the like, can be blended herein.

It is believed that the beneficial results obtained by this invention are attributed to the discovery of a concentration range of the metal salts of fatty acids which appears to have a surprising efiectin that a more unifonn degree of crystallinity isobtainedlt is further believed that the improvement in gloss shownherein is not due to the presence of the metal salts on the surface of the film. This hypothesis is in sharp contrast to the known use of these materials in larger amounts to achieve decreased blocking and increased slip. It is further believed that the use of the'larger amounts (i.e., about 1,000 p.p.m. or

greater) results in an exudation of the metal salts to the surface of the film which accounts for the increased slip and decreased blocking and also impairs the printability. Also it has been found that concentrations of zinc fatty acid salts greater than 400 p.p.m. adversely affects the heat-scalability range of articles such as films fabricated from the ethylene polymers. For the purposes of this invention, heat-scalability of an article of ethylene polymer is defined as the ease which the article can be fused to itself or another identical or similar ethylene polymer to form a strong bond or seal. In order for an article of ethylene polymer to have good heat-scalability, it is necessary that the spread between the fusion temperature of the polymer and the temperature at which the-heat sealing apparatus'will pass through the article, so-called bum-through temperature, be aslarge as possible. If the bum-through temperature is reached or approached during heat sealing, the resultis often an opening orcut in the article in the sealed truded from a die onto chilled rollers and the film is collected on a take up roll.

The following examples are presented solely to illustrate this invention. No limitation on the scope of the invention is intended nor implied. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 Polyethylene having a melt index of 1.0 (ASTM D-l238-65 T) and a density of 0.926 gms/cc (ASTM D-792-66) was used to make a master-batch of polyethylene and zinc stearate. Portions of this masterbatch were then extruded with more polyethylene to give the concentrations of zinc stearate set forth below in the Table I.

The control sample of polyethylene was reextruded also to assure that it had the same heat-ageing history as' the test samp es.

The blends were then extruded into a 1.25 mil film onto a chilled roll at a rate of 225 feet per minute and the film collected on a take up roll.

The clarity of these films was measured and found to be substantially unaffected.

TABLE 1 Run p.p.m. Haze, Gardner Gloss, Percent No. Zn Stearate Percent 20 Head 45 Head C, (Control) 4.5 76 79 l 27 3.9 93 84 2 100 3.2 97 85 3 300 3.3 100+ 86 B, 500 3.2 100+ 84 Not an example of the invention.

The table shows that small amounts of zinc stearate provide an increase in gloss and a decrease in haze. As indicated by Run No. 8,, larger amounts of this additive do not contribute any worthwhile gain, and adversely affect the heat sealability.

In a similar manner blends were extruded into 1.25 mil film onto a chilled roll at a rate of 100 feet per minute and were electrostatically treated in line with a Lepel Model HFSG-Z spark generator manufactured by Lepel Laboratories, Inc. The films are passed over a Mylar-covered metal grounded roll with a high tension treater bar positioned over the roll with about a 60 mil gap so that the film passes between the bar and the roll. A power input is established between the bar and the roll in the range of 100 to 15,000 watts. The passage of current causes a corona effect in the air space next to the film surface. This corona discharge treats the film so that the film surface will accept and bond to various inks.

The printability of the film was evaluated by spraying a single pass of Poly-Treat Check Ink (manufactured by the lndependent lnk., Gardena, Calif.) on the film and allowing it to dry for 5 minutes. A strip of pressure sensitive tape was then pressed onto the ink surface and stripped away. The ink adhesion was then estimated by comparing the amount of ink remaining on the film with the amount that came ofi on the tape. The results in percent ink adhesion are recorded below.

TABLE 11 Power Input for p.p.m. Zn Stearate Corona Treatment 0 300 500 1000 1500 2000 5,100 watts 50 90 95 90 75 70 7,600 watts 50 80 90 90 70 70 10,000 watts 50 60 90 80 60 70 12,600 watts 70 60 50 70 50 60 In order to illustrate the adverse effects of heat ageing which might be encountered during extreme conditions of storage, portions of the same films were subjected to heat ageing at 140 F. for 66 hours prior to the ink application. The results in percent ink adhesion are recorded below.

TABLE 111 Power Input for p.p.m. Zn Stearate Corona Treatment 0 300 500 1000 1500 2000 5,100 watts 40 40 30 5 5 0 7,600 watts 60 70 60 20 5 10,000 watts 90 80 90 40 30 12,600 watts 70 90 80 40 30 20 Table 11 thus illustrates that the printability of the electrostatically treated film is considerably increased when a small amount of zinc stearate is added while at the same time the gloss is increased (Table I). It is also apparent from Table III that the additive, when used in the amounts claimed herein, does not substantially change the printability of heat-aged electrostatically treated film.

EXAMPLE 2 By repeating the procedures of Example 1 with a polyethylene having a melt index of 10.0 and a density of 0.934 similar results were obtained. It was noted that as the concentration of zinc stearate exceeded about 500 p.p.m. no further improvement in haze or gloss was obtained. Furthermore, when concentrations greater than about 500 p.p.m. are used the ink adhesion is adversely affected and this effect is more pronounced with heat ageing.

EXAMPLE 3 A blend of three polyethylenes was made by extruding together 50 parts by weight of a polyethylene having a density of 0.934 (melt index 10.0), 30 parts of a polyethylene having a density of 0.921 (melt index 1.2), and 20 parts of a polyethylene having a density of 0.934 (melt index 0.85). The blend was modified by the addition of 1,600 p.p.m. of finely divided silica and 800 p.p.m. of erucamide as slip and block agents and 150 p.p.m. of 2,6-di-tert-butyl-4-methylphenol as an antioxidant. Finally, a portion of the blend was modified by the addition of 250 p.p.m. of zinc stearate, the remainder of the blend being retained as a control. When this blend was cast into films and treated as in Example 1 it was found that the haze and gloss were considerably improved over the control having no zinc salt additive.

EXAMPLE 4 The procedures set forth in Example 1 are repeated using a copolymer of percent by weight of ethylene and 20 percent by weight of ethyl acrylate. Similar improvement in gloss and printability of the electrostatically treated film are observed.

EXAMPLE 5 Example 1 is repeated with a terpolymer of 5 percent by weight of vinyl acetate, 1.5 percent of propylene and 93.5 percent of ethylene. Again the gloss and ability to be rendered printable are increased over the unmodified control.

EXAMPLE 6 Polyethylene having a melt index of 2 [ASTM D-l238-65 (E)] and a density of 0.921 gms/cc (ASTM D-792-66) was used to make a masterbatch of polyethylene and zinc stearate. Portions of this masterbatch were then extruded with more of the polyethylene to give blends having the concentrations of zinc stearate set forth below in Table IV.

A control sample (C,) of the polyethylene was reextruded also to assure that it had the same heat-ageing history as the test samples.

The blends and the control sample were then extruded into 1.5 mil films onto a chilled roll at a rate of feet per minute and the films were collected on take-up rolls.

The heat-scalability of the films of the blends and the control sample was determined by measuring the fusion temperature and the bum-through temperature for each film. The results are recorded in Table IV.

TABLE IV Heat-Seal Range, F. Run p.p.m. Fusion Burn Through No. Zn Stearate Temperature Temperature (1) C, 0 220 300 1 300 220 307 2 400 220 287 3 500 220 233 4* 600 218 233 Not an example of the invention.

( l Average of three specimens.

As evidenced by the results of Table IV, the compositions of Example 6 have a heat-scalability range of 67 to 87 F. (Runs 1 and 2); or at least about 60 P, which is equivalent to the heat scalability range of the polyethylene containing no zinc stearate whereas compositions containing 500 p.p.m. or more of zinc stearate have a very poor heat scalability range.

Results similar to the foregoing may also be obtained when the zinc stearate is substituted by the analogous zinc fatty acid salts hereinbefore indicated.

I claim:

1. An ethylene polymer having improved surface gloss and decreased haze and having a heat-sealability range of at least about 60 F., said polymer having incorporated therein from about 20 parts per million to about 400 parts per million of a zinc salt of a fatty acid having from 7 to 22 carbon atoms.

2. A polymer as set forth in claim 1 wherein the zinc salt is incorporated in an amount from about 100 to about 350 parts per million.

3. A polymer as set forth in claim 1 wherein the fatty acid has a chain length in the range of 16 to 20 carbon atoms.

4. A polymer as set forth in claim 1 in which the polymer is polyethylene.

5. A polymer as set forth in claim 1 in which the polymer is a terpolymer of ethylene, propylene and vinyl acetate.

6. A polymer as set forth in claim 1 in which the polymer is a copolymer of ethylene and ethyl acrylate.

7. A polymer as set forth in claim 1 in which the zinc salt of a fatty acid is zinc stearate.

8. A polymer as set forth in claim 1 in which the zinc salt of a fatty acid is zinc stearate and in which the ethylene polymer is polyethylene. 

2. A polymer as set forth in claim 1 wherein the zinc salt is incorporated in an amount from about 100 to about 350 parts per million.
 3. A polymer as set forth in claim 1 wherein the fatty acid has a chain length in the range of 16 to 20 carbon atoms.
 4. A polymer as set forth in claim 1 in which the polymer is polyethylene.
 5. A polymer as set forth in claim 1 in which the polymer is a terpolymer of ethylene, propylene and vinyl acetate.
 6. A polymer as set forth in claim 1 in which the polymer is a copolymer of ethylene and ethyl acrylate.
 7. A polymer as set forth in claim 1 in which the zinc salt of a fatty acid is zinc stearate.
 8. A polymer as set forth in claim 1 in which the zinc salt of a fatty acid is zinc stearate and in which the ethylene polymer is polyethylene. 